Sparsed U-TDOA wireless location networks

ABSTRACT

In an overlay, U-TDOA-based, Wireless Location System, LMUs typically co-located with BTSs, are used to collect radio signaling both in the forward and reverse channels. Techniques are used to compensate for sparse LMU deployments where sections of the U-TDOA service area are uplink demodulation or downlink beacon discovery limited.

CROSS REFERENCE

The subject matter described herein relates to the subject matterdescribed in the following co-pending applications: U.S. Pat.application Ser. No. 11/736,868, filed Apr. 18, 2007; U.S. Patentapplication Ser. No. 11/736,902, filed Apr. 18, 2007; and U.S. Patentapplication Ser. No. 11/736,950, filed Apr. 18, 2007.

TECHNICAL FIELD

The present invention relates generally to methods and apparatus forlocating wireless devices, also called mobile stations (MS), such asthose used in analog or digital cellular systems, personalcommunications systems (PCS), enhanced specialized mobile radios(ESMRs), and other types of wireless communications systems. Moreparticularly, but not exclusively, the present invention relates tomethods for reducing the receiver deployment density of a WirelessLocation System (WLS) and thereby reducing the overall cost of such adeployment.

BACKGROUND

A. Wireless Location

Early work relating to Wireless Location Systems is described in U.S.Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,”which discloses a system for locating cellular telephones using timedifference of arrival (TDOA) techniques. This and other exemplarypatents (discussed below) are assigned to TruePosition, Inc., theassignee of the present invention. The '144 patent describes what may bereferred to as an uplink-time-difference-of-arrival (U-TDOA) cellulartelephone location system. The described system may be configured tomonitor control channel transmissions from one or more cellulartelephones and to use central or station-based processing to compute thegeographic location(s) of the phone(s). TruePosition and others havecontinued to develop significant enhancements to the original inventiveconcepts. An example of a U-TDOA WLS is depicted in FIG. 1. As shown,the system includes four major subsystems: the Signal Collection Systems(SCS's) 10, the TDOA Location Processors (TLP's) 12, the ApplicationProcessors (AP's) 14, and the Network Operations Console (NOC) 16. EachSCS is responsible for receiving the RF signals transmitted by thewireless transmitters on both control channels and voice channels. Ingeneral, an SCS (now sometimes called an LMU, or Location MeasuringUnit) is preferably installed at a wireless carrier's cell site, andtherefore operates in parallel to a base station. Each TLP 12 isresponsible for managing a network of SCS's 10 and for providing acentralized pool of digital signal processing (DSP) resources that canbe used in the location calculations. The SCS's 10 and the TLP's 12operate together to determine the location of the wireless transmitters.Both the SCS's 10 and TLP's 12 contain a significant amount of DSPresources, and the software in these systems can operate dynamically todetermine where to perform a particular processing function based upontradeoffs in processing time, communications time, queuing time, andcost. In addition, the WLS may include a plurality of SCS regions eachof which comprises multiple SCS's 10. For example, “SCS Region 1”includes SCS's 10A and 10B that are located at respective cell sites andshare antennas with the base stations at those cell sites. Drop andinsert units 11A and 11B are used to interface fractional T1/E1 lines tofull T1/E1 lines, which in turn are coupled to a digital access andcontrol system (DACS) 13A. The DACS 13A and another DACS 13B are usedfor communications between the SCS's 10A, 10B, etc., and multiple TLP's12A, 12B, etc. As shown, the TLP's are typically collocated andinterconnected via an Ethernet network (backbone) and a second,redundant Ethernet network. Also coupled to the Ethernet networks aremultiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminalserver 15. Routers 19A and 19B are used to couple one WLS to one or moreother Wireless Location System(s).

FIG. 1A depicts the components representative of a standard wirelesscommunications system (WCS) 100, which may take the form of a cellulartelephone network or the like. Although the technology represented inFIG. 1A is expressed with some of the terminology typical of a GlobalSystem for Mobile Communications (GSM) infrastructure, the technology isalso comparably applicable to and beneficial for implementations ofcellular wireless communications in accord with other standards, such asthe Third Generation Partnership Project (3GPP) technical specificationsdescribing the Universal Mobile Telecommunications Service (UMTS). InFIG. 1A, the wireless mobile communications unit or mobile station (MS)101 communicates via a radio frequency (RF) link carrying transmissionsto and from a base transceiver station (BTS) 102. As highlighted in thedashed circle in FIG. 1A, the BTS facilities include the uplink-receive(U_Rx) and downlink-transmit (D_Tx) antenna(s) and associated cables forthe appropriate signals carrying the wireless communications. A set of(typically three) BTS cell sectors (or sectorized cellular areas ofoperation) cover a localized communications area or cell (surrounding aserving BTS) served by the antenna(s) deployed at the BTS terminallocation. Each cell sector is identified by its unique cell globalidentifier (CGI, which term is also used herein to refer to the BTS cellfacilities). Each BTS may individually or independently generate itstime base or time-standard/reference for its transmitted downlinksignals based upon an independent oscillator that operates at a nominaltime base frequency, within specification tolerances. For GSM service, acompliant standard BTS timebase reference is specified to operate at 13MHz, within a tolerance of 0.05 ppm or 0.65 Hz. A set of the variousBTSs covering a broader operational region are controlled by a basestation controller (BSC) 103. The BSC manages the MSs and BTSs operatingwithin its domain, and this management includes the handover of theresponsibility for the integrity of the RF link with a particular MSfrom one BTS to another, as the MS moves from the cellular coverage ofthe cells of one BTS to those of the other BTS. In a similar manner at alower level of communications management, the BSC also manages thehandover of an MS from one BTS sector to another and the BTS detects thesuccessful execution of the handovers within its domain. At a higherlevel of management, a mobile switching center (MSC) 104 manages amultiplicity of BSCs. In supporting the WCS operations, any MS operatingunder the control of its particular serving CGI (SCGI) is used tosynchronize itself to the SCGI's transmitted BTS downlink “beacon”signal, and thus the signals from the distinct BTSs are not required tobe synchronized to a common time standard, such as the GPS time base.

FIG. 1B shows a WLS that cooperates as an adjunct to a wirelesscommunications system. In this example, the WLS is called a ServingMobile Location Center (SMLC) 110. An infrastructure-based, or“overlay,” WLS can be represented with the overlay configuration ofcomponents depicted in FIG. 1B. In FIG. 1B, the RF uplink signals in thecommunications channel from the MS/UE 101 of interest are received andmeasured by LMUs 112 that are deployed at locations distributedthroughout the operational domain of the communications system. (Noteregarding terminology: In 3GPP GSM terminology, the term “SMLC” refersto the entire WLS whereas in other contexts “SMLC” refers to thesub-system component that is called a “WLP”. As also used herein, the3GPP term “LMU” refers to the geographically dispersed SMLC/WLScomponent that receives transmitted RF signals and measures (e.g.,location-related) signal characteristics, whereas such a component maybe called the signal collection system “SCS” in other contexts ordescriptions of the background art.) Typically, as may be visualizedwith the “overlay” of FIG. 1B on top of FIG. 1A, LMUs 112 are deployedat BTS 102 facilities, and thus the LMU usually accesses or “taps” itsuplink-receive (U_Rx) signals for the location-related measurements viamulti-coupling to the same signal feeds that the BTS uses from theantenna(s) deployed for the communications. For time basesynchronization of the (location-related) data collections andmeasurements at the distributed LMU sites, the LMU accesses GPS signalsvia a GPS-receive (GPS_Rx) antenna with cable, as highlighted in thedashed circle in FIG. 1B. Additionally, the LMU senses the BTS downlinktransmissions via a downlink-receive (D_Rx) antenna with cable. Asdepicted in FIG. 1B, although the LMUs are typically but not necessarilydeployed at BTS sites, they are also not necessarily deployedone-for-one with the BTSs. The measurements of the received signalcharacteristics extracted by multiple LMUs are managed and collectedthrough wireless location processors (WLPs) 203, each of which directsthe operations of multiple LMUs. The WLP oversees the selection of theparticular LMUs that are tasked with providing the measurements for aparticular MS of interest. Upon reception of the appropriately measuredsignal data, perhaps including through other WLPs managing LMUs notunder its direct control, the WLP will typically also evaluate the dataand determine the optimal (location) estimate based upon the data.Typically, a WLP may manage the operations of LMUs covering a geographicregion for which the corresponding communications services are providedby multiple BSCs. The wireless location gateway (WLG) 114 of the SMLCconducts overall control and tasking of the WLPs. The WLG is typically(but not necessarily) co-located with a MSC 104 (and may interface withit). The WLG interfaces with and exchanges location-related requests,information, or data with the multiple BSCs it serves within thecommunications system. The WLG validates the location-service requests,and disperses the location-determination results to authorizedrecipients.

The performance of a U-TDOA WLS (and other location systems) is normallyexpressed as one or more circular error probabilities. The United StatesFederal Communications Commission (FCC), as part of the Enhanced 9-1-1Phase II mandate, requires that network-based systems, such as a U-TDOAsystem, be deployed to yield a precision that generates a one-hundredmeter (100 m or 328.1 feet) accuracy for 67% of emergency servicescallers and a three-hundred meter (300 m or 984.25 feet) accuracy for95% of emergency services callers. The requirements for precision varywith the location service deployed, but if the precision (such aspredicted by the Cramer-Rao bound for instance) of the U-TDOA locationsystem is such that the location quality of service is exceeded by adeploying fewer LMUs than BTSs, such a deployment would be advantageousbecause it would reduce the cost of the system.

The inventive techniques and concepts described herein apply to time andfrequency division multiplexed (TDMA/FDMA) radio communications systemsincluding the widely used IS-136 (TDMA), GSM, and OFDM wireless systems,as well as code-division radio communications systems such as CDMA(IS-95, IS-2000) and Universal Mobile Telecommunications System (UTMS),the latter of which is also known as W-CDMA. The Global System forMobile Communications (GSM) model discussed above is an exemplary butnot exclusive environment in which the present invention may be used.

B. Problems with Building a Sparse WLS

In a non-sparsed U-TDOA system (a U-TDOA system with 1 LMU per BTS),LMUs are able to detect and demodulate downlink signals (beacons orBroadcast Control Channels (BCCH)) from the resident cell. The measuredtiming is then compared to system time, determined by the LMU'sGPS-based clock, and then sent to the SMLC for storage or forwarding toother LMUs. Each LMU will then be able quickly to demodulate uplinkmessaging since the channel and timeslot are provided in the locationrequest and the frame timing offset from system time for each adjacentcell and sector is known.

In a sparsed U-TDOA system (a U-TDOA system with a less than 1 LMU perBTS deployment ratio), the increased distances between radio emitter(the mobile device) and the radio receiver (the LMU) resulting from theselective deployment (“sparsing”) will have an adverse effect on U-TDOAlocation accuracy and will inhibit the LMU's ability to determine frametiming offsets, which are needed in a GSM environment. An LMU, togenerate the timestamps needed for TDOA, should: (1) detect anddemodulate cell downlink beacons to determine cell timing, and (2)detect and demodulate uplink signals. The requirements that the LMUreceive and demodulate both uplink and downlink signals in the presenceof noise, adjacent channel interference, co-channel interference and atthe distance of several cell radii make it difficult to minimize LMUdeployment cost.

SUMMARY

The following summary is intended to explain several aspects of theillustrative embodiments described in greater detail below. This summaryis not intended to cover all inventive aspects of the disclosed subjectmatter, nor is it intended to limit the scope of protection of theclaims set forth below.

In an overlay, U-TDOA-based, Wireless Location System, LMUs typicallyco-located with BTSs are used to collect radio signaling both in theforward and reverse channels. When LMUs are not deployed at each BTSsite, a sparse deployment, beacon reception and uplink reception canlimit the performance and service area of the U-TDOA system. A goal ofthe present invention is to provide a method and system for minimizingLMU deployment costs. Illustrative embodiments provide a number oftechniques to minimize the cost of a U-TDOA deployment via sparsing.These techniques may be applied as shown in FIG. 3 to reduce thedeployment ratio of LMUs to BTSs and thus the overall cost of the U-TDOAwireless location system.

The present invention may be embodied as an iterative method fordesigning a sparse wireless location system (WLS), and as a softwaretool for use in performing the iterative design method. For example, inone exemplary embodiment, the iterative method includes performing anintelligent network design process to produce an initial network design;performing a preliminary network design analysis to determine that atleast one of the following performance limiting factors affects theinitial network design: downlink beacon discovery, accuracy, and uplinkdemodulation; and modifying the initial network design based on theperformance limiting factor determined as affecting the initial networkdesign.

The WLS may comprise a U-TDOA system including a plurality ofgeographically dispersed location measuring units (LMUs), and the WLSmay be overlaid on a GSM wireless communications system comprising aplurality of geographically dispersed base transceiver stations (BTSs).In addition, a presently preferred implementation of the iterativemethod further comprises identifying at least one cluster ofco-synchronized cell sectors prior to performing the intelligent networkdesign process.

In an illustrative embodiment, the method further comprises adding atleast one LMU to the network design based on a determination that theperformance limiting factor affecting the initial network design isaccuracy. The illustrative embodiment may also include removing at leastone LMU from the network design based on a determination that noperformance limiting factor affects the initial network design.

When the performance limiting factor is downlink beacon discovery, themethod may also include deploying at least one enhanced downlinkantenna, deploying downlink interference cancellation, deploying BTSsynchronization, adding at least one LMU to the network design, or acombination of any of these. In addition, when the performance limitingfactor is downlink beacon discovery, the method may also includedetermining whether an Abis monitoring system (AMS) is deployed, and ifnot deploying at least one downlink-only LMU at an identified site. Ifan AMS is deployed, the method may include enabling the use of EnhancedBeacon Synchronization (EBS) and AMS-derived beacon timing functions.

When the performance limiting factor is uplink demodulation, the methodmay further include determining whether communications systemdemodulation data is enabled, and if so enabling a demodulated datafeature, and if not determining that an AMS is not deployed and enablinga mid-amble only correction feature. In addition, if communicationssystem demodulation data is not enabled, the method may includedetermining that an AMS is deployed and enabling an AMS-deriveddemodulated data feature. When the performance limiting factor affectingthe initial network design is uplink demodulation, the method mayfurther comprise adding at least one LMU to the network design, and/oradding dedicated antenna facilities. It should also be noted thatobtaining the demodulation bits from a link monitoring system, e.g., anAMS, can reduce the cost and complexity of an LMU, i.e., even innon-sparsed environments.

Other aspects of the embodiments disclosed herein are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description arebetter understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary constructions of the invention; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 schematically depicts a Wireless Location System.

FIG. 1A depicts a representative configuration of the major componentsof a wireless communications system (WCS). FIG. 1B shows arepresentative configuration of the major components of an overlay WLS,sometimes called the serving mobile location center (SMLC).

FIG. 2 schematically depicts a GSM/GPRS reference model.

FIGS. 3A-3G are or provide a flowchart showing a progression oftechniques that may be used to sparse a U-TDOA system based on theresults of pre-installation analysis, simulation modeling and fielddetermined empirical results.

FIG. 4 illustrates a process and messaging for Beacon Discovery, whichmay be used for a beacon-only LMU.

FIG. 5 illustrates a sparsed TDOA network and is referenced below inexplaining that the TDOA hyperbola's width is due to timing errorsbetween LMU clocks and unresolvable signal timing caused by multipathradio propagation. These errors may be multiplied by the GDOP.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We will now describe illustrative embodiments of the present invention.First, we provide a detailed overview of the problem and then a moredetailed description of our solutions.

A. Overview

In an effort to reduce the cost of a U-TDOA system, receivers may beinstalled at a subset of the cell sites in the communications network.As discussed above, in a U-TDOA system having one LMU per BTS, each LMUcan detect and demodulate downlink beacons or Broadcast Control Channels(BCCH) from its resident cell (e.g., its co-located BTS). The measuredtiming may then be compared to system time as determined by the LMU'sGPS-based clock, and then sent to an SMLC for storage or forwarding toother LMUs. This enables each LMU to demodulate uplink messaging.

In a sparse U-TDOA system (a U-TDOA system with a less than 1 LMU perBTS deployment ratio), the increased distances between radio emitter(the mobile device) and the radio receiver (the LMU) resulting from theselective deployment (“sparsing”) will have an adverse effect on U-TDOAlocation accuracy as SNR decreases with distance, co-channelinterference increases, and effects of GDOP are magnified. Additionally,in a sparsed U-TDOA system, LMUs are required not only to determine theradio signal timing of the resident cell and sectors, but also anysurrounding cells and sectors that do not have a resident LMU. Thisability to receive and demodulate the beacons of non-resident cells andsectors in proximity to the LMU is used to determine frame timingoffsets.

The a priori knowledge of the frame timing is used for U-TDOA locationof GSM devices such as mobile phones. Each GSM frequency channel isshared by up to 8 mobile stations. Since there are a maximum of eightusers per frequency, there are eight timeslots (TS) per GSM frame.Therefore, each mobile uses the channel for one timeslot and then waitsfor its turn to come round again in the next frame. The mobiletransmitter turns on only during its active timeslot. The requirement totransmit in a single timeslot and stay idle during the remaining seventimeslots results in very tight demands on the mechanism for on/offswitching of the RF power. If a mobile station does not performaccording to the specifications, it will disturb other mobile stationsin adjacent timeslots and on adjacent channels. The WLS taskinginformation from a location request contains channel information,supplied by the wireless communications system or by added monitoringsubsystems, that includes the timeslot assigned to themobile-of-interest; but without the frame timing information derivedfrom the beacon discovery process, there is no simple way for the LMU toreliably distinguish one timeslot from another.

The inability to detect the beacons from surrounding cells and sectorsmeans that the frame timing cannot be derived in advance of a locationrequest, thus the LMU location rate is severely reduced by the need tocollect long periods of radio energy from the assigned channel,eliminating the ability of the WLS to perform most control channellocations and resulting in higher failed locations due to calls handingoff during the signal collection phase of the U-TDOA location.

An LMU, to generate the timestamps needed for TDOA, should: (1) detectand demodulate cell downlink beacons to determine cell timing, and (2)detect and demodulate uplink signals. The downlink signals will be fromadjacent cells and sectors and from those potentially able to be servedby the LMU. The uplink signals can be destined for the resident cell orfor any serving cells in near proximity, and can originate from anypoint served by those cells. The requirements that the LMU receive anddemodulate both uplink and downlink signals in the presence of noise,adjacent channel interference, co-channel interference and at thedistance of several cell radii make it difficult to minimize LMUdeployment costs. A goal of the present invention is to provide amulti-pronged technique for minimizing such LMU deployment costs.

To summarize, beacon discovery is a problem due to:

-   -   Co-channel interference    -   Adjacent channel interference    -   Receiver saturation    -   Directional antennae deployments    -   Antenna Downtilt    -   Near-far effects.

When deploying LMUs in less than a 1:1 ratio to BTSs in GSM networks inurban areas, we have observed that the limiting factors are Beacon(BCCH) Discovery and Uplink Demodulation, and not location accuracy.Uplink Demodulation is a problem since the successful measurement ofTDOA values relies upon a “clean” (high SNR, low phase noise, lowinterference, etc.) reference signal with which measured signals frommultiple sites are correlated to provide an estimate of the TDOA betweenthe reference signal and the signal received at each site (see U.S. Pat.Nos. 5,327,144; 6,047,192; 6,400,320; 6,483,460; and 6,661,379). Inuplink demodulation limited areas, no signal of sufficient quality isavailable at any LMU to act as the reference signal.

Uplink demodulation is a problem due to:

-   -   Co-channel interference    -   Adjacent channel interference    -   Receiver saturation    -   Directional antennae deployments    -   Antenna Downtilt    -   Near-far effects    -   Path loss due to radiative propagation loss, signal absorption        and diffraction losses, and multipath signal corruption.

TruePosition has developed a number of techniques to minimize the costof a U-TDOA deployment via sparsing. These techniques are applied asshown in FIGS. 3A-3G to reduce the deployment ratio of LMUs to BTSs andthus the overall cost of the U-TDOA wireless location system. FIGS.3A-3G are flowcharts of an exemplary implementation of a process inaccordance with the present invention. The illustrated steps aresummarized below.

-   -   Step 300: Begin sparsing design process.    -   Step 301: Identify clusters of co-synchronized cell sectors.    -   Step 302: Perform intelligent network design process.    -   Step 303: Perform preliminary network design analysis.    -   Step 304: Determine performance limiting factor:        -   (A) downlink beacon discovery—go to FIGS. 3B, 3C;        -   (B) accuracy—go to FIG. 3D;        -   (C) none—go to FIG. 3E, Remove LMU(s) (Step 321); or        -   (D) uplink demodulation—go to FIGS. 3F and 3G.

As shown in FIG. 3B, in a “downlink beacon discovery limited” situation,the following steps are carried out:

-   -   Step 305: Deploy enhanced downlink antenna.    -   Step 306: Deploy downlink interference cancellation.    -   Step 307: Deploy BTS synchronization.    -   Step 308: Add additional LMU(s) to the service area.        In addition, at Step 310, the process includes determining        whether an AMS (Abis monitoring system) is deployed (see FIG.        3C). If not, it proceeds to step 311. If so, it proceeds to step        312.    -   Step 311: Deploy downlink-only LMUs at identified sites.    -   Step 312: Enable use of EBS (Enhanced Beacon Synchronization)        and AMS-derived beacon timing functions.

As shown in FIGS. 3B and 3C, both of these processes are followed byStep 309: Perform intelligent network design process again, this timewith updated design specifications.

As shown in FIG. 3D, in an “accuracy limited” situation, the followingsteps are carried out:

-   -   Step 313: Increase Signal Integration Time;    -   Step 314: Deploy TDOA/ECID Hybrid;    -   Step 315: Deploy TDOA/AoA Hybrid;    -   Step 308: Add additional LMU(s) to Service Area.

FIGS. 3F and 3G depict the process steps for the “uplink demodulationlimited” scenario. The steps include:

-   -   Step 316: Determine whether communications system demodulation        data is enabled. If not, go to Step 310; if so, go to Step 319.    -   Step 319: Enable demodulated data feature.    -   Step 317: Enable mid-amble only correction feature.        Step 318: Enable AMS-derived demodulated data feature.        (Obtaining the demodulation data from an AMS can reduce the cost        and complexity of the LMU. This is a benefit even when sparsing        is not an issue.)

Also, as shown in FIG. 3G, for the “uplink demodulation limited” case,the process includes:

-   -   Step 308: Add additional LMU(s) to the service area.    -   Step 320: Add dedicated antenna facilities.

Here again, as shown in FIGS. 3F and 3G, these steps are followed byStep 309: Perform intelligent network design, with updated designspecifications.

In subsection C., below, we will describe our inventive techniques ingreater detail. First, however, we provide a non-limiting discussion ofthe GSM Reference Model, which provides an exemplary (although notexclusive) and suitable context in which embodiments of the presentinvention may be used.

B. GSM Network Reference Model

FIG. 2 depicts a GSM Network Reference Model (this figure is amodification of the 3GPP standardized generic LCS logical architectureof GSM 03.71, Revision 8.9.0 section 5.6). We will now discuss thisreference model in order to provide further context for the remainingdescription of presently preferred embodiments of our inventivetechnology. Our summary description of the GSM Network Reference Modelis by no means intended to suggest that our invention is limited tosystems conforming to the GSM Network Reference Model. The followingparagraphs summarize the elements depicted in FIG. 2:

210 BTS—In a GSM system, the BTS (Base Transceiver Station) terminatesthe GSM radio interface. Each BTS includes a number of TRX(Transceivers), amplifiers, filters, and antenna. The term BTS includesthe power, environmental shelter and environmental controls required tohouse electronic devices. The BTS connects to the MS (Mobile Station)via the Um radio interface and to the BSC via the Abis interface.

220 U-TDOA LMU—The LMU (Location Measurement Unit) makes radiomeasurements to support U-TDOA and is typically co-located with the BTSallowing for joint use of the radio antenna and facilities. All locationand assistance measurements obtained by an LMU are supplied to aparticular SMLC associated with the LMU. Instructions concerning thetiming, the nature and any periodicity of these measurements are eitherprovided by the SMLC or are pre-administered in the LMU. Thegeographically distributed U-TDOA LMU is connected to the SMLC by adedicated connection.

223 Le Interface—The 3GPP standardized Le interface (The OMA/LIF MobileLocation Protocol 3.2.1 as standardized in 3GPP TS 23.171) is used bythe LBS application (LCS Client) to communicate with the GMLC forrequesting locations, and receiving location responses. Servicesprovided include: Standard Immediate Location, Emergency ImmediateLocation, Standard Location Reporting, Emergency Location Reporting, andTriggered Location Reporting.

224 Lb Interface—The Lb interface is a standardized messaging interfacethat enables communication between a BSC and an SMLC. Through thisinterface, the GSM network triggers location requests directly to theWLS, which then obtains additional channel data from the BSC to completethe location process. This location information is then routed by theGSM network to the requesting or assigned LBS application. The Lbinterface is available using either SS7 or SIGTRAN transport. The Lbinterface is optional if the AMS with all associated interfaces andprobes are installed. Both the AMS and Lb may operate in the samenetwork.

225 A Passive Tap—The AMS is interfaced to the A interface via the useof passive taps. Typical implementation of the passive tap is byreplication of the interface messaging via a Digital Cross-Connection orDigital Access Exchange (DAX).

226 Abis Passive Tap—The AMS is interfaced to the Abis interface via theuse of passive taps. Typical implementation of the passive tap is byreplication of the interface messaging via a Digital Cross-Connection orDigital Access Exchange (DAX).

227 HLR—The HLR (Home Location Register) is a database within the HPLMN(Home Public Land Mobile Network). The HLR is responsible for themaintenance of user subscription information. The HLR provides routinginformation for MT (Mobile Terminated) calls, SMS (Short MessageService). The HLR provides cell/sector information for location requestrouting and Any Time Interrogation (ATI) operations.

228 GSM SCF—The gsmSCF (GSM Service Control Function) defines theintelligent network (IN) control environment for every call that invokesan IN service. The gsmSCF also stores the service logic associated withIN services. For Location-based services, the gsmSCF uses the Lcinterface for interconnection to the GSM MAP network. The Lc interfaceis applicable only in CAMEL phase 3 and 4. The procedures and signalingassociated with the gsmSCF are defined in GSM 03.78 (now 3GPP TS 23.078)and GSM 09.02 (now 3GPP TS 29.002), respectively. Locations related toIN functions of the gsmSCF include interrogation on subscriber locationusing the ATI (Any Time Interrogation) and ALR (Active LocationRetrieval) procedures.

229 E5+ Interface—The E5+ interface 229 is based on the E5 interfacedescribed in the ANSI/ESTI standard J-STD-036 “Enhanced Wireless 9-1-1Phase II”. This interface between the SMLC's WLG component and GMLC,allows the GMLC to request location directly from the SMLC. The E5+interface also allows the SMLC to push autonomously developed locationsdirectly to the GMLC for caching or immediate delivery over the Leinterface to an LBS application.

230 MSC—The MSC (Mobile Switching Center) contains functionalityresponsible for MS subscription authorization and managing call-relatedand non-call related positioning requests of GSM LCS. The MSC isaccessible to the GMLC via the Lg interface and the SMLC via the Lsinterface. If connected to SGSN through the Gs interface, it checkswhether the mobile station is GPRS attached to decide whether to pagethe mobile station on the A or Gs interface.

231 Lg interface—The 3GPP standardized Interface between a GMLC (GatewayMobile Location Center) and the VMSC (Visited Mobile Location Center).

232 Lh Interface—The 3GPP standardized Interface between a GMLC and theHLR. Communications using this interface take place over the GSM-MAPNetwork.

233 Lc Interface—The 3GPP standardized Interface between a GMLC and thegsmSCF communications using this interface take place over the GSM-MAPNetwork.

238 GSM MAP Network—The SS7-based network, using the MAP protocol, whichenables real time access, routing, and communication between thedistributed nodes of a mobile cellular network.

240 BSC—The BSC (Base Station Controller) is the functional entitywithin the GSM architecture that is responsible for RR (Radio Resource)allocation to a Mobile Station, frequency administration and handoverbetween BTS controlled by the BSC. For a U-TDOA location system, the BSCsupplies the SMLC with radio channel information and characteristics.The BSC is connected to the BTS via the Abis interface, the MSC via theA interface and to the SMLC via the Lb interface.

250 AMS—The AMS (A/Abis Monitoring Subsystem) is described inTruePosition's U.S. Pat. No. 6,782,264, Aug. 24, 2004, “Monitoring ofCall Information in a Wireless Location System,” and further expanded inU.S. Published Patent Application 20060003775, filed Jun. 10, 2005,“Advanced Triggers for Location-based Service Applications in a WirelessLocation System.” The AMS (or LMS) passively monitors the Abis and/or Ainterfaces for location triggering events, messaging and subscriberinformation allowing the SMLC to perform autonomous (from theperspective of the wireless communications system) U-TDOA, CGI, CGI+TAand ECID location calculations. The AMS is connected to the SMLC via adigital communications link. The AMS, with all associated interfaces andprobes, is optional if the Lb interface is installed. Both the AMS andLb may operate in the same network.

260 SMLC—The Serving Mobile Location Center (SMLC) containsfunctionality used to support LCS. In one PLMN, there may be more thanone SMLC. The SMLC manages the overall coordination and scheduling ofresources used to perform positioning of a MS. It also calculates thefinal location estimate and accuracy. The SMLC supports positioning viasignaling on the Lb interface to the BSC serving the target MS. The SMLCmay support the Lp interface to enable access to information andresources owned by another SMLC. The SMLC controls a number of LMUs forthe purpose of obtaining radio interface measurements to locate or helplocate MS subscribers in the area that it serves. The SMLC isadministered with the capabilities and types of measurement produced byeach of its LMUs. Signaling between the SMLC and U-TDOA LMU istransferred via a dedicated digital connection. A digital connection tothe AMS and the E5+ interface to the GMLC allows the SMLC and LMUs toproduce autonomous locations based on the AMS provided triggering andradio information and push autonomous locations to the GMLC.

270 WLP—The Wireless Location Processor (WLP) component of the SMLCcluster integrates information from the served LMUs to calculate theposition of the caller or wireless device, using the single or multipletechnologies as selected by the WLG. The WLP connects to the served LMUsand the WLP via digital communications links.

280 WLG—The Wireless Location Gateway (WLG) communicates with thewireless network, receiving requests for location, determining the bestlocation method for the application, and sending the location recordback to the network. The connection to the wireless network may bepassive using the AMS or may be active, using the Lb interfaceinterconnection to the BSC.

290 Abis—The Abis Interface is the GSM standardized signaling interfacebetween the BTS and BSC.

295 A—The A interface is the standardized interface in the GSM networkarchitecture between the BSC and an MSC. The interface supports channelsfor signaling and traffic.

296 GMLC—The GMLC (Gateway Mobile Location Center) contains theAuthentication, Access Control, Administration and Accountingfunctionality used to support Location-based services (LBS) (also knownas LCS (Location Services)). In one PLMN (Public Land Mobile Network),there may be more than one GMLC. The GMLC is the first node an externalLBS or LCS client accesses in a GSM or UMTS network. The Gateway MobileLocation Center (GMLC) capabilities are defined in the followingstandards: GSM 03.71 (Location Services (LCS)—functional description),3GPP TS 23.271 (Functional stage 2 description of LCS), MobileApplication Part Protocol (3GPP TS 09.02 “MAP”) and CAMEL (3GPP TS23.079). Additional functionality GMLC functionality includes:

-   -   Location Client Control Function (LCCF): The Location Client        Control Function (LCCF) manages the external interface towards        multiple Application Server/Location Client Function (LCF). The        LCCF identifies the LCS client within the wireless operator by        requesting client verification and authorization (i.e. verifies        that the LCS client is allowed to position the subscriber)        through interaction with the Location Client Authorization        Function (LCAF). The LCCF handles mobility management for        location services (LCS) e.g., forwarding of positioning requests        to VLR. The LCCF determines if the final positioning estimate        satisfies the QoS for the purpose of retry/reject. The LCCF        provides flow control of positioning requests between        simultaneous positioning requests. It may order the Location        Client Coordinate Transformation Function (LCCTF) to perform a        transformation to local coordinates. It also generates charging        and billing-related data for LCS via the Location System Billing        Function (LSBF).    -   Location Client Authorization Function (LCAF): The Location        Client Authorization Function (LCAF) is responsible for        providing access and subscription authorization to a client.        Specifically, it provides authorization to a LCS client        requesting access to the network and authorizes the subscription        of a client. LCAF provides authorization to a LCS client        requesting Location Information of a specific MS.    -   Location System Billing Function (LSBF): The Location System        Billing Function (LSBF) is responsible for charging and billing        activity within the network related to location services (LCS).        This includes charging and billing of both clients and        subscribers. Specifically, the LSBF collects charging related        data and data for accounting between PLMNs.    -   Location System Operations Function (LSOF): The Location System        Operations Function (LSOF) is responsible for provisioning of        data, positioning capabilities, data related to clients and        subscription (LCS client data and MS data), validation, fault        management and performance management of the GMLC.    -   Location Client Coordinate Transformation Function (LCCTF): The        Location Client Coordinate Transformation Function (LCCTF)        provides conversion of a location estimate expressed according        to a universal latitude and longitude system into an estimate        expressed according to a local geographic system understood by        the LCF and known as location information. The local system        required for a particular LCF would either be known from        subscription information or explicitly indicated by the LCF.

297 LBS—The LBS application (LCS Client) can initiate location requeststo the GMLC and receive location responses from the GMLC. When an AMShas been deployed as part of the WLS, the LBS application may be allowedto pre-configure triggering events, messaging, or subscriber informationon the AMS to enable autonomous passive location.

C. Implementing a Sparse U-TDOA Network

To deploy LMUs at the minimum number of sites while retaining adesignated level of U-TDOA performance, the sparsing process shown inFIGS. 3A-3G may be performed. In the following subsections, we addressthe following topics in greater detail: base station timing analysis;intelligent system design for sparsing; predicted coverage area,predicted site density, and predicted area of responsibility; downlinkcoverage requirements and secondary sector coverage requirements;preliminary system design for sparsing analysis; downlink beacondiscovery limited performance; uplink demodulation limited performance;improving downlink beacon discovery limited performance, enhanceddownlink antenna for improving downlink beacon discovery limitedperformance, and link monitoring for improving downlink beacon discoverylimited performance; enhanced beacon sync; downlink only LMUdeployments; improving uplink demodulation limited performance, linkmonitoring for improving uplink demodulation limited performance, knownsequence correlation for improving uplink demodulation limitedperformance; and alternative embodiments.

Base Station Timing Analysis (See Step 301 in FIG. 3A)

Once the performance parameters have been established and the relevantwireless system data has been collected, but before the preliminarysystem design can be completed, the wireless network timing sourcesshould be evaluated. In a TDMA-based system, such as GSM, basetransceiver stations (BTSs) are normally unsynchronized, i.e., basestations are deployed without a common clock reference. The accuracyrequirements for GSM base transceiver stations were formulated by theETSI organization (European Telecommunications Standards Institute) inthe GSM 05.10 recommendation “Radio subsystem synchronization” asfollows:

5.1 The BS shall use a single frequency source of absolute accuracybetter than 0.05 parts-per-million (ppm) for both RF frequencygeneration and clocking the time base. The same source shall be used forall carriers at the BS.

As a result of this requirement, channels internal to a single CGI aresynchronized. (A CGI can be a cell—in the case of an omni-directionalantenna, or a sector of a cell—in the case of directional antennas.) Dueto the difficulty of large scale geographical BTS deployments based on acommon system clock reference, no requirement exists for synchronizationof channels between other GSM BTSs. GSM base stations have traditionallyderived their required frequency accuracy by locking a crystaloscillator within the base station to a recovered clock signal from aT1/E1 line backhaul facility. Timing signals based on a primaryreference source (PRS) transmitted over the backhaul keep the embeddedoscillator calibrated to within sufficient accuracy.

Although not a GSM requirement, due to equipment deployments and designchoices by manufacturers, clusters of commonly timed, co-synchronizedsectors and occasionally adjacent cells may exist in the wirelesslocation system's service area. Also not required by the GSMspecification but widely available to GSM operators after theintroduction of the U.S. Air Force deployment of the NavStar GlobalPosition System (GPS) satellite navigation system, GSM systems can bemade co-synchronous by timing derived from the GPS radio signal andmessaging. Equivalent timing abilities are expected to be available fromany global or regional satellite navigation system.

Further information concerning BTS synchronization may be found inInternational Patent Application WO06088472A1, filed on Apr. 25, 2005,“Base Transceiver Station (BTS) Synchronization.” This documentdescribes how, in a network overlay wireless location solution for a GSMor UMTS communications network, spectrum utilization can be made moreefficient by synchronizing the BTSs, which can require distributing atiming signal to all BTSs, or installing a satellite-based timing unitin each site. In an example of this solution, LMUs are installed at someor all of the BTS sites for the purpose of locating wireless devices.The LMUs are used to measure the timing of various uplink and/ordownlink signals in the cellular network in support of various locationtechniques. These LMUs may include a GPS-based timing reference module,which may be used to synchronize the time bases of all LMUs. To reducethe overall cost of BTS synchronization, the LMU distributes timingsignals, including a periodic electrical pulse as well as timedescription information, on a serial or other interface, which isavailable for other nodes to use for synchronization. The format of theelectrical pulse and time description information is modified throughhardware and software to adapt to the various formats required byvarious BTS types. For example, BTSs with co-located LMUs can receive asynchronization signal with little or no hardware cost. An ExternalInterface Unit (EIU) may be used to adapt to various BTS hardwareformats. For BTS sites not equipped with an LMU, a Timing MeasurementUnit (TMU) can be used. The TMU has the single function of providing BTStime signals in the same formats as provided by the LMUs. The timesignals provided by the TMUs are synchronous to the signals provided bythe LMUs. This timing-only TMU has a lower cost than the LMU because itdoes not support the uplink or downlink signal measurement functions.This approach allows a cellular operator to synchronize BTSs at arelatively low cost.

Once the timing analysis of the BTSs and thus the radio channels in theservice area has been completed, a map of downlink channel framing canbe created. When the overall service area timing analysis is complete, apreliminary deployment design can be performed.

Intelligent System Design for Sparsing (See Step 302 in FIG. 3A)

TruePosition, Inc., the assignee of the present invention, makes anIntelligent System Design tool. The system planning application providesautomated LMU site selection during the market design process. Thisfeature incorporates criteria-based selection of LMU sites in a marketwith less than a 100% deployment ratio of LMUs to BTSs (a “sparsed”system).

The Intelligent System Design tool automatically selects the set of LMUsites that will provide the best location performance. To do this, thesystem planning software tool orders carrier base stations by redundancymetrics, and then removes one site at a time with the lowest redundancymetric, unless downlink coverage requirements or secondary sectorcoverage requirements are not satisfied for that site. Redundancymetrics are recalculated after each removal. Removal of sites continuesuntil a target LMU deployment ratio is achieved or until the site poolis exhausted.

A redundancy metric for a site is obtained by multiplying several basicmetrics for a site:Redundancy Metric=(Coverage Area)^(K)(Site Density)^(L)(Area ofResponsibility)^(M) where K=0.5, L=1, M=1.

Note that the constants K, L, and M have been determined empirically.

Coverage Area: A base station coverage area is an approximate area insquare kilometers where the base station sectors can be used ascooperators in a WLS. The area is computed by finding a distance wherecertain threshold power is achieved. The power computation is based on asophisticated radio propagation/path loss model (such as the extendedCOST231-Hata model). Therefore, the antenna parameters that contributeto coverage area computations are:

height (agl) (the higher the site the better coverage)

height above mean sea level (amsl) (used to come up with effectiveheight)

vertical beam width (the less the value the better coverage)

horizontal beam width

tilt (the closer to 0 the better, tilt of 10 can severely reducecoverage, for example)

antenna gain (the more the gain the better coverage)

number of sectors.

The antenna parameters are necessary to account for individualproperties of antennas.

Site Density: The site density is an average number of sites per squarekilometer in the vicinity of a base station of interest. This valuetakes into consideration only sites that are closer than R kilometersfrom a base station. The R is chosen as a distance to a 20^(th) closestsite. For the system planning tool computations only initial (before anyremoval) site densities are used. The initial site density correlateswith the environment where base station is installed. For example,urban, suburban, and rural environments will have different sitedensities.

Area of Responsibility: This is the area that bounds a region (a Voronoiregion), each point of which is closer to the current base station thanto any other base station. Area of responsibility is recalculated aftereach base station removal from a configuration. This recalculationfacilitates uniform distribution of sites and better geometry ofcooperators for the U-TDOA calculation.

Downlink coverage requirements: The Intelligent System Design toolshould make sure that after removal of an LMU from a carrier's site map,the site's downlink channel(s) can still be well monitored by downlinkantennas installed on remaining LMU sites. These requirements includeminimum downlink SNR that can include some safety margin and minimumnumber of downlink antennas that should be able to monitor calls handledby a non-LMU tower. To check these requirements the tool uses aPropagation Model that accounts for terrain loss. The program, dependingon co-synchronous setting of the network, interprets the requirementsdifferently. In a network that is generally unsynchronized (for exampleGSM), if two or more cell sectors (CGIs) are synchronized such that theyhave the same relative frame timing and frame numbers, then those cellsectors are said to be co-synchronous. This is sometimes present in GSMnetworks by making all cell sectors at a given site (typically 2, 3, or6) co-synchronous to each other.

Secondary sector coverage requirements: A “secondary sector” is asector/CGI other than the serving sector that may still be capable ofdemodulating the uplink signal from the mobile station. The primary andsecondary sectors are all tasked with demodulation of the uplink signalto provide redundancy. Secondary sector coverage requirements make surethat in each representative point of an accuracy grid one can find asufficient number of secondary sectors. These requirements includeminimum SNR to be a secondary, and percentages of points that have 0, 1,2 and 3 secondary sectors. To check these requirements the IntelligentSystem Design tool uses an original Propagation Model that accounts forterrain loss and carrier supplied coverage polygons. The IntelligentSystem Design tool allows the operator to create an a priori U-TDOAsystem design (the baseline design) that uses less than one LMU persite. This allows the Operator to deploy the minimum number of LMUs forany required level of accuracy and save costs associated with un-neededLMU deployments.

Initial Baseline Design (FIG. 3A, Steps 300, 301, 302)

The Intelligent System Design tool is the tool that defines which sitescan be left un-deployed in a sparse LMU deployment scenario.

With a desired sparing target deployment ratio (a ratio of less than oneLMU per base station), the Intelligent System Design tool will be usedto identify the sites that should be deployed with those LMUs to achievethe best system performance resulting in a system design. This design iscalled the initial baseline design. This initial baseline design maycontain areas that are beacon discovery limited, uplink demodulationlimited or accuracy limited. In each iteration of the sparsing designprocess, a new candidate design is developed.

The Intelligent System Design tool works by creating, for every point inthe geographic service area, the set of TDOA baselines from everypotential LMU site within or in proximity to the service area. Thenumber of potential TDOA baselines for any point using the LMUspotentially involved in a TDOA location (as determined by the predictedreceived signal strength from the radio propagation model) is given bythe formula:Maximum No. of TDOA Baselines=(No. of LMUs)*(No. of LMUs−1)*(½), when(No. of LMUs)≧3

The factor that is limiting the deployment ratio of a network can beidentified in a progression of steps.

The first item to check is downlink beacon discovery. This can beanalyzed by considering the transmit power of each beacon and the pathloss of the downlink signal from the transmit antenna to each site thatis a candidate to have an LMU deployed. This will yield a received powerlevel at each LMU. Based on the receiver sensitivity characteristics, itcan be determined whether or not each LMU can discover a given downlinkbeacon. As long as every beacon can be discovered by at least one LMU(or more if redundancy is required), then the design is not downlinkbeacon discovery limited. If any beacon cannot be discovered by at leastone LMU, then the system design is limited by this factor, and LMUsshould be added to this design until this situation is resolved.

Once all the beacons can be discovered by at least one LMU, the nextlimiting factor, Uplink Demodulation, can be assessed. Based on thereceiver sensitivity of the base station, and the path loss to differentareas served by that site, the minimum transmit power of the mobileuplink signal needed to maintain this link can be determined at eachlocation. Based on this mobile uplink transmit power, and similar pathloss calculations, the received power level at surrounding LMU sites canbe determined. If this received power is greater than the minimum signalstrength needed by the LMU to demodulate the signal at at least one sitethat is a candidate to have an LMU deployed, then the system design isnot Uplink Demodulation limited. If there are areas where the mobile canbe served by a cell site, but the uplink signal does not propagate toany LMU site with sufficient power levels to allow demodulation, thenthe system is Uplink Demodulation limited, and LMUs should be added tothis design until this situation is resolved.

Once all the beacons have been discovered, and all areas serviceable bythe cell sites in the design can also be demodulated by the deployedLMUs, a final check can be made to determine if the system design isAccuracy limited. This entails first determining the minimum mobileuplink transmit power used to maintain the link at the locations servedby the cell sites in the design. From this transmit power, and the pathloss to all surrounding LMU sites, the received signal power at each ofthe surrounding LMUs can be determined. If this signal level is greaterthan the TDOA detection sensitivity level, which is significantly lowerthan the Demodulation sensitivity level, then that LMU is considered acooperating LMU for the location of mobiles from this area. All suchcooperating LMUs are identified. The terrain and density of sites in theregion are used to estimate the multi-path induced spread in the TDOAmeasurements. Based on the geometry of these cooperating LMUs, and themulti-path spread, the estimated location accuracy for this area can becomputed. This process is repeated for all the areas served by the cellsites in the design to produce an aggregate location accuracy for theentire design. If this accuracy level meets the requirements of thedesign, then the system design is not Accuracy limited. If the estimatedaccuracy level falls short of the requirements of the design, then thesystem is Accuracy limited and additional LMUs should be added to thisdesign until this situation is resolved.

Revising the Initial Baseline Design (FIG. 3A, Step 303)

Introduction: Adding LMUs to the WLS Candidate Design

If the initial baseline design or candidate design contains areas withinthe defined service area with the defined geographic service area thatare beacon discovery limited, uplink demodulation limited or accuracylimited, then the initial or current sparsing ratio should be decreasedand LMU added to the initial baseline design.

Adding LMUs is performed on an LMU-by-LMU basis. First, the performancelimitation and the geographic area that is performance limited isidentified. The available base stations not currently hosting LMUs areidentified with the affected area or in geographic proximity to theaffected area are noted (if no unused base station is available in theaffected area, alternative siting arrangements, for example cell sitesused by other wireless carriers or other radio services, can beconsidered). For each of these potential sites, the system tool will beused identify the next best site for an LMU to be added using thetechniques described.

Adding LMU(s) for Beacon Limited (FIG. 3B, Step 308)

When an area is beacon limited, the system planning tool is used topredict the beacon discovery list for each potential LMU site. Eachpotential LMU site's predicted beacon discovery list is then comparedwith the list of beacons that are not predicted to be discovered by theexisting LMU population. LMUs are added to the design until all beaconsare discoverable and the amount of redundancy (the number of times abeacon is discovered by multiple LMUs) in the beacon list is minimized.

Adding LMUs for Uplink Demodulation Limited (FIG. 3G, Step 308)

When an area is Uplink Demodulation Limited, the system planning tool isused to predict the uplink demodulation performance for each potentialLMU site. Each potential LMU site's predicted uplink demodulation areais then compared with area not sufficiently covered by the existing LMUpopulation. LMUs are added to the design until the area that was uplinkdemodulation limited is eliminated and any overlapping coverage from theLMUs in proximity to the affected area is minimized.

Adding LMUs for Accuracy Limited (FIG. 3D, Step 308)

When an area is Accuracy Limited, the system planning tool is used topredict the improvement in system accuracy performance for eachpotential LMU site. On a site-by-site basis, the system planning tooldevelops an accuracy prediction for the entire service area based onthat site being added to the entire LMU population already present inthe current candidate design. If addition of a single site does notimprove accuracy performance sufficiently, the process is repeated foreach pair of potential LMU sites. This process of adding LMUs andevaluation of the predicted accuracy in each new potential networkdesign is repeated until the accuracy performance threshold is reachedor until all potential LMU sites are occupied by LMUs.

If the list of potential LMU sites are exhausted, then additionalalternative siting arrangements, for example cell sites used by otherwireless carriers or other radio services or standalone LMU sites withdedicated facilities, can be considered.

Introduction: Removing LMUs from the WLS Design (FIG. 3E, Step 321)

If the initial baseline design of candidate design does not containareas within the defined geographic service area that are beacondiscovery limited, uplink demodulation limited or accuracy limited, thenthe sparsing ratio may be able to be increased and LMU(s) removed fromthe initial baseline design or candidate design.

To delete LMUs from a design, the system planning tool will be usedidentify the next best LMU to remove. All LMUs in the candidate designare considered. The decision to remove an LMU from the initial baselinedesign is based on redundancy—for both beacons and accuracy.

Removing LMU(s) without Degrading Beacon Limited Performance

The first step in determining potential LMUs for removal from the designis examination of the beacon lists for each LMU in the design. Thesystem planning tool is used to predict beacons that would be discoveredby each LMU. The system planning tool is then used to determine if anyof the predicted beacons are predicted to be discovered by other LMUs.If all the beacons discovered by an LMU are also discovered by others,then this LMU is a candidate to be removed. It is the level of beaconredundancy determines which LMU get removed first from the design. Theremoval of LMUs from the design can be repeated, barring theintroduction of the other sparsing related performance issues (accuracy,uplink demodulation, etc.) until redundancy of beacon discovery isminimized. In an ideal, maximally sparsed system, there would be nobeacon redundancy.

Please note that the determination of beacon redundancy can be performedin a deployed system from examination of the LMU received beacon listand in cases of system optimization or wireless network reconfiguration,the actual beacon performance can be used in place of that determinedfrom the theoretical propagation model.

Removing LMU(s) without Degrading Uplink Demodulation Performance

The next step in determining potential LMUs for removal from the designis examination of the uplink signal strengths for each LMU in thedesign.

The identification and LMUs that can be removed from the updated designbased on Uplink Demodulation Performance is done using the radiopropagation model created for the initial baseline design, alreadyaltered to reflect the removal of LMUs based on redundancy in beacondiscovery. This updated model has minimized the beacon discoveryredundancy and initially has no Uplink Demodulation Performance limitedareas.

The received signal strengths at all LMUs for all possible transmissionpoints in the service area are examined in this stage. If successful(strong enough to be demodulated) signal reception is predicted at twoor more LMUs, then reception is said to be redundant. If the set ofsignals predicted to be received and demodulated by a specific LMU arecompletely redundant, that LMU may be removed from the current designbarring the introduction of the other sparsing related performanceissues (accuracy and beacon discovery).

Please note that the determination of Uplink Demodulation redundancy canbe performed in a deployed system from examination of the LMU receivedsignal records and in cases of system optimization or wireless networkreconfiguration, the actual uplink demodulation performance can be usedin place of that determined from the theoretical propagation model.

Removing LMU(s) without Degrading Accuracy Performance

Accuracy limited should be thought of in terms of meeting certaintargets (e.g., the FCC Phase II mandate for network-based locationsystem) for accuracy numbers. The system is accuracy limited if thecurrent design does not meet the required accuracy targets. I.e.,additional sparsing by removal of LMUs from the candidate design cannotbe done because accuracy requirements are not being met.

In a sparsed WLS, not limited by Beacon discovery or Uplink Demodulationperformance, the primary determinant of Accuracy Limited areas is theHorizontal Geometric Dilution of Precision (HDOP or GDOP)

A relationship exists between the location error, measurement error andgeometry. The effect of the geometry is represented by a scalar quantitythat acts to magnify the measurement error or dilute the precision ofthe computed result. This quantity is referred to as the HorizontalDilution of Precision (HDOP) and is the ratio of the Root-Mean-Square(RMS) position error to the RMS measurement error σ. Mathematically, itcan be written as:

${HDOP} = \sqrt{\frac{\sigma_{n}^{2} + \sigma_{e}^{2}}{\sigma^{2}}}$

where σ_(n) ² and σ_(e) ² is the variances of the horizontal componentsfrom the covariance matrix of the measurements. Physically, the bestHDOP is realized when the intersection of the TDOA hyperbolas betweenbaseline LMU pairs is orthogonal. An ideal HDOP situation arises whenthe emitter is at the center of a circle and all of the receiving sitesare uniformly distributed about the circumference of the circle.

Determination of likely LMUs for removal in a candidate system thatmeets or exceeds accuracy requirements is done through the examinationof the system planning tool generated accuracy plots of the service areaand the TDOA baselines, between each pair of LMUs, generated for eachpoint in the service area.

The system planning tool considers both the redundancy in predicted TDOAhyperbolic baselines and the degree of orthogonality in the baselines.Redundant baselines do not contribute to increased accuracy andtherefore can be eliminated. Baselines with low degrees of orthogonalitycan actually magnify the inaccuracy of a measurement and must thereforebe minimized. If an LMU produces TDOA hyperbolic baselines with lowdegrees of orthogonality, it can be removed and the WLS accuracyperformance recalculated.

Please note that evaluation of the accuracy performance of a deployedsystem can be determined from examination of calculated location versusknown actual location for test transmissions. In cases where a deployedsystem is suffering from accuracy limited areas, that information can bebrought into the radio propagation model and a new baseline designcalculated. From the new baseline, the entire intelligent design processcan be reiterated and potential sites for the addition or deletion ofLMUs determined.

Preliminary System Design for Sparsing Analysis (See Step 303 in FIG.3A)

The preliminary system design for sparsing analysis is used to determineif TDOA performance limiting factors exist for the intelligent systemdesign produced by the design planning and evaluation application. Theability to deploy LMUs in a less then one-to-one ratio to the cell sitesin a network (sparse deployment) is limited by three main factors:Downlink Beacon Discovery, Uplink Demodulation, and Accuracy. These arerepresented in FIGS. 3B-C (Downlink Beacon Discovery Limited), FIGS.3D-E (Accuracy Limited), and FIGS. 3F-G (Uplink Demodulation Limited).

The factor that is limiting the deployment ratio of a network can beidentified in progressive steps.

The first item to check is the downlink beacon discovery. This can beanalyzed by considering the transmit power of each beacon, the path lossof the downlink signal from the transmit antenna to each site that is acandidate to have an LMU deployed. This will yield a received powerlevel at each LMU. Based on the receiver sensitivity characteristics, itcan be determined whether or not each LMU can discover a given downlinkbeacon. As long as every beacon can be discovered by at least one LMU(or more if redundancy is required), then the design is not downlinkbeacon discovery limited. If any beacon can not be discovered by atleast one LMU, then the system design is limited by this factor, andLMUs should be added to this design until this situation is resolved.

Once all the beacons can be discovered by at least one LMU, the nextlimiting factor, Uplink Demodulation, can be assessed. Based on thereceiver sensitivity of the base station, and the path loss to differentareas served by that site, the minimum transmit power of the mobileuplink signal needed to maintain this link can be determined at eachlocation. Based on this mobile uplink transmit power, and similar pathloss calculations, the received power level at surrounding LMU sites canbe determined. If this received power is greater than the minimum signalstrength needed by the LMU to demodulate the signal at at least one sitethat is a candidate for to have an LMU deployed, then the system designis not Uplink Demodulation limited. If there are areas where the mobilecan be served by a cell site, but the uplink signal does not propagateto any LMU site with sufficient power levels to allow demodulation, thenthe system is Uplink Demodulation limited, and LMUs should be added tothis design until this situation is resolved.

Once all the beacons have been discovered, and all areas serviceable bythe cell sites in the design can also be demodulated by the deployedLMUs, a final check can be made to determine if the system design isAccuracy limited. This requires first determining the minimum mobileuplink transmit power used to maintain the link at the locations servedby the cell sites in the design. From this transmit power, and the pathloss to all surrounding LMU sites, the received signal power at each ofthe surrounding LMUs can be determined. If this signal level is greaterthan the TDOA detection sensitivity level, which is significantly lowerthan the Demodulation sensitivity level, then that LMU is considered acooperating LMU for the location of mobiles from this area. All suchcooperating LMUs are identified. The terrain and density of sites in theregion are used to estimate the multi-path induced spread in the TDOAmeasurements. Based on the geometry of these cooperating LMUs, and themulti-path spread, estimated location accuracy for this area can becomputed. This process is repeated for all the areas served by the cellsites in the design to produce aggregate location accuracy for theentire design. If this accuracy level meets the requirements of thedesign, then the system design is not Accuracy limited. If the estimatedaccuracy level fall short of the requirements for the design, then thesystem is Accuracy limited and additional LMUs should be added to thisdesign until this situation is resolved.

Downlink Beacon Discovery Limited

The first performance limiting factor is Downlink Beacon Discovery.Locating mobile stations on a GSM network using U-TDOA techniquesrequires knowledge of the GSM frame timing used by the mobile station.The frame timing of the mobile is defined by the frame timing broadcastby each sector in its Downlink BCCH channel. In general, each cellsector within the GSM network has independent frame timing. When LMUsare deployed at every cell site, each LMU acquires the frame timing ofthe cells at that site by decoding the BCCH transmitted by those cells.This process (as shown in FIG. 4) is called Beacon Discovery. Whensparse deployment is used, the frame timing of the cell where LMUs arenot deployed should be discovered by LMUs at neighboring sites. If theratio of LMUs deployed gets too low, then there will be cells for whichno LMU is capable of discovering the beacons for these cells. In thatcase, the MSs placing calls served by those cells with undiscoveredbeacons may not be located. This deployment is said to be DownlinkBeacon Discovery Limited.

For the sake of completeness, the Beacon Discovery process illustratedby FIG. 4 will now be summarized. As shown, the process includes thefollowing steps:

-   -   1. The MS transmits an Access Burst on the RACH. This mobile        originated call is initiated in the CGI of interest and any of        the reference CGIs. Any MS requesting a dedicated channel will        trigger this process.    -   2. The RACH signal is received by a BTS and the BTS sends a        Channel Required message to the BSC and AMS (Abis monitoring        system). The Channel Required message contains the RFN data of        the target CGI.    -   3. The Location Gateway (LG) sends an RFN Sync Query message to        the AMS, and the AMS responds with an RFN Sync Response, which        contains a set of CGI, ARFCN, RFN, and Abis TS data.    -   4. The LG then sends an Enhanced Sync Monitor (AFRCN int)        message to the LMUs, which are tasked to find 51 multiframe        boundaries, relative to GPS time, on the beacon channel        provided.    -   5. The LMUs respond to the LG with a GSM Sync Report        (syncType=enhanced) message. The LG performs a final calculation        to find an adjusted RFN mapping to GPS timestamp. This mapping        is recorded in a synchronization table of the LG.    -   6. The process is repeated for other CGIs of interest.

Uplink Demodulation Limited

The second performance limiting factor is the ability to demodulate theuplink transmission from the mobile station. This is needed to derive areference signal that is then used to make the TDOA measurements at thecooperating LMUs.

When LMUs are deployed at every cell site, the LMU at the site on whichthe call or non-call related messaging is transmitted can easilydemodulate the uplink signal. When sparse deployment is used, then theuplink signal for calls placed on cells where LMUs are not deployedshould be demodulated by LMUs at neighboring sites. If the ratio of LMUsdeployed gets too low, then there will be cells for which no LMU iscapable of demodulating uplink signals related to calls placed on thatcell. In that case, calls placed on those cells may not be located. Thisdeployment is said to be Uplink Demodulation Limited.

Accuracy Limited

In some cases, the U-TDOA deployment can be Accuracy Limited even thoughnot Uplink Demodulation Limited or Downlink Beacon Discovery Limited.The cause of accuracy limited U-TDOA deployments is primarily theGeometric Dilution of Precision (GDOP). Common to all multi-laterationsystems, the GDOP arises in the wireless TDOA LMU deployment from theshallowness of the angles at which the TDOA-generated hyperbolaintercept. If the effect of the GDOP multiplier renders the locationerror in an area beyond design specifications, then the area is AccuracyLimited. FIG. 5 shows an illustrative example of a sparsed U-TDOAnetwork with the TDOA hyperbolas widened by the timing and measurementerrors induced by the radio multipath environment and the various timingand measurement errors in a real U-TDOA system.

Additional LMU deployments beyond the service area may be used to lowerthe GDOP within the service area, thus removing the Accuracy Limitedareas. Other techniques for dealing with Accuracy Limited areas in aU-TDOA system include the addition of hybrid location technologies.

FIG. 3D, Step 314, shows the addition of Enhanced Cell-ID (ECID) to theU-TDOA system to compensate for the predicted Accuracy Limited areas. Ina U-TDOA system, fallback to in situ wireless network-based locationtechniques is possible. These network-based location techniques includeusing the Cell-ID or Cell-ID with sector to generate a location based onthe SMLC's knowledge of the underlying geography of the service area andthe topology of the wireless network.

The use of radio propagation delay information (“timing advance” or“round-trip-time”) and mobile-generated beacon power measurements can beeffective (if available), with the SMLC's knowledge of the BTS beaconpower levels, to refine a basic Cell-ID/sector location. This technique,Enhanced Cell ID (ECID), is a potential accuracy improvement over thebasic Cell-ID techniques. ECID location is achieved by using additionalTiming Advance (TA) and Power Measurement (PM) information derived fromthe wireless network to create a location. Use of cell-id (CGI) andtiming advance (TA) as a fall-back is inherent in ECID calculationssince both the CGI and TA are available to the SMLC regardless of thenumber or usefulness of beacon power measurements available in theNetwork Measurement Report (NMR). These network-based approaches tolocation are known to those skilled in the art as are the statisticalmethods used in the attempt to improve accuracy using historical usagedata and radio propagation models.

In a sparse U-TDOA deployment, U-TDOA coverage is expected to providesufficient performance over the majority of the service area. However,location coverage holes (areas of insufficient or non-existent accuracy)may exist due to the limitations of downlink beacon discovery, uplinkdemodulation issues inherent in a sparse U-TDOA network, and the fickle,if not capricious, nature of the radio environment.

One remedy to a coverage hole (a geographic area where the TDOA systemis accuracy limited) is the installation of an additional LMU. Thisapproach, which is shown at Step 308 of FIG. 3G as well as Step 308 ofFIG. 3D (Accuracy Limited), will raise the deployment (LMU:BTS) ratio.If this approach is not satisfactory to the wireless operator, a hybridU-TDOA/ECID system may be deployed. In addition to potentially curingthe lack-of-location-coverage issue, deployment of the hybridU-TDOA/ECID system allows for location quality-of-servicedifferentiation for offered location-based services (LBS) applications.ECID is especially useful for LBS applications requiring low to mediumaccuracy with periodic updates, such as tracking while the mobile deviceis on conversation state.

Since the signal collection for an ECID location is performed at themobile device using the higher-powered forward (BTS-to-mobile device)channel, ECID performance is independent of the performance limitingfactors resulting from the sparsely deployed reverse-channel collectingU-TDOA deployment.

GSM ECID is a cell and sector (CGI) based approach coupled with a range(Timing Advance (TA) or Round-Trip-Time (RTT)) from the serving cellsite and a power-difference-of-arrival measurement (PDOA)). In additionto the serving cell, the sector (if any) and the timing advance, thetransmit power of each beacon in the network should be known and thelocation of each serving sector (transmit antenna) should be known bythe SMLC to correctly calculate the PDOA. As an alternative to the PDOAcalculation, a database of beacon strengths for a calibrated grid can beused with pattern matching. The grid within a serving cell or sector maybe calibrated by recording mobile beacon reception patterns or bysophisticated radio propagation models.

ECID is also a method uniquely suited for medium accuracy location onthe border area between differing U-TDOA service areas or networks.Border areas are where U-TDOA accuracy is likely to be poor due to widecell spacing and poor network topology, which resulting in high GDOP.The wide cell spacing results from operator inclinations to put borderareas in sparsely served areas. The poor network topology is the resultof the differing U-TDOA networks being deployed on a sharp linearboundary. Borders in U-TDOA service areas can result from operatorcommunications network deployment boundaries, inability of U-TDOAnetworks to share LMU-developed TDOA information, or the operator'selection to use two or more vendors to provide U-TDOA-based wirelesslocation systems.

Since, with ECID, the mobile receiver acts as the point of signalcollection, beacon radio power can be collected by the mobile from boththe serving network and the adjacent network. The SMLC awareness of theadjacent network transmitter locations and frequencies, as collected bythe serving network LMU-based downlink beacon receivers, allows for theadjacent network received beacons to be used for the ECID locationcalculation without reliance on interoperating LMUs based on theadjacent area or network.

FIG. 3D, Step 315, shows the addition of AoA to the U-TDOA system tocombat the accuracy limited performance. See U.S. Pat. Nos. 6,108,555(Aug. 22, 2000) and 6,119,013 (Sep. 12, 2000), both or which areentitled “Enhanced Time Difference Localization System”.

Improving Downlink Beacon Discovery Limited Performance

In un-synchronized networks, such as GSM or UMTS, in which thetransmission time offsets of the signals radiated by one base stationrelative to another are unknown, LMUs should monitor beacon timing todetermine the frame timing. When LMU deployments are less than 1:1(BTS:LMU), the timing of a cell's radio transmissions should bedetermined not by a resident LMU but rather should be determined from anLMU in an adjacent or further cell.

The following are techniques to facilitate downlink beacon discovery insparsed deployments.

Enhanced Downlink Antenna (Step 305 in FIG. 3B)

When the performance limiting factor is Downlink Beacon Discovery, thenthe first and least expensive option is the addition of enhanceddownlink antennae to the LMU sites identified in proximity of theperformance limited areas. Use of an enhanced downlink receive antennaallows the LMU to better detect and demodulate the beacon (the BCCH inGSM) broadcasts from surrounding cells and sectors in a sparseddeployment. Deployment of the downlink antenna can be accomplished viadirect mounting to the LMU, but antennas mounted on base stationexteriors or on the cell tower provide less attenuated environments andtherefore better reception.

However, downlink antennas can suffer from too little attenuation aswell as too much. Downlink receivers can suffer from the fact that thebeacons are transmitted at such high power. For example, if an LMU(along with it's receive antenna) is located at or near one BTS, thebeacons from that BTS will be received at very high power. Because ofthe nature of the GSM waveform, a significant portion of the energy fromthose beacons spillover into the adjacent frequency channels. If nearbysites that do not have LMUs located at them (sparsed sites) have theirbeacons transmitting on one of these adjacent channels, then thespillover from the strong local beacon can make it very difficult todetect and demodulate the weaker beacon from the remote site.

In addition to the adjacent channel problem, in some instances (likeroof top deployments where the LMU downlink receive antenna is placed inclose proximity to the carrier's transmit antenna) the local beacons areso strong that they saturate the front end of the LMU, thus making itimpossible to detect any remote beacons even if they are not on adjacentchannels. In this scenario, success may be had by introducing aninexpensive line attenuator to reduce the received signal level at theLMU's downlink receiver so that it is no longer driven into saturation.Line attenuation can result in many remote Beacons being discovered thatwere previously undiscovered.

Besides optimizing physical antenna placement and the application ofline attenuators, a third technique, that of the application ofprogrammable notch filters to the antenna feed, can be used. Byfiltering out the frequency(s) from the co-located, host BTS, thesaturation issue is mitigated without degradation of remote beaconreception. In this approach, programmable filters are necessary due tothe mercurial nature of the operator's frequency planning and the needto adjust to new frequency allocations that include the host BTS.

Link Monitoring for Enhanced Beacon Synchronization (Step 312 of FIG.3C)

As disclosed in U.S. Pat. No. 6,782,264, Aug. 24, 2004, “Monitoring ofCall Information in a Wireless Location System,” and further expanded inU.S. Published Patent Application 20060003775, filed Jun. 10, 2005,“Advanced Triggers for Location-based Service Applications in a WirelessLocation System,” an Abis Monitoring System (AMS) or Link MonitoringSystem (LMS) can be deployed in conjunction with the wireless locationsystem to supply a passive means of triggering the location system. Ascost savings measures, an overlay LMS may be deployed to monitor theAbis (BTS-to-BSC) link only or the required LMS functionality may beincorporated directly into the BSC. The deployment of the AMS or LMSfunctionality allows for certain techniques allowing for lower LMUdeployment densities.

The Enhanced Beacon Synchronization feature employs an LMS or AMS tomonitor the Abis links of the BTS units involved, and to access the GSMFrame Number information quickly and reliably, reducing latency andimproving system throughput. This enhanced synchronization techniqueincreases the system sensitivity to discovering GSM beacons and theirrespective mapping to GPS time. Using this technique, the Abis monitorwill provide synchronization information that will partially describethe mapping of the absolute frame number (FN) to GPS time. Theparameters provided by the LMS contain the RFN (reduced frame number,T1′, T2, T3), the partial description of GSM frame number. Thisinformation will be combined with observations and calculations madedirectly by LMUs monitoring the downlink path to further converge on thetiming solution.

Specifically, “beacon synchronization” is the method by which the systemdetermines the absolute frame time reference used by a particularCGI—the absolute time reference is used to determine the super-framesequence as a function of time. A U-TDOA system relies upon preciseknowledge of the timing of the Frame Number (FN) to properly collect thefrequency-hopped signals on the correct channel at the correct time.Enhanced beacon synchronization uses coarse Frame Number (FN)measurements from the LMS, combined with a detection process on thedownlink, which allows beacon timing to be measured at signal-to-noiseratios (SNRs) that are 11 dB lower than with normal downlink beaconmonitoring. This improved sensitivity allows beacons to be detected bymore distant LMUs, facilitating more sparse LMU deployment.

The general function of determining absolute frame timing referencebegins during the basic Beacon Synchronization process. The LMU performsa four-step process to derive the timing synchronization between GSMframes and GPS time:

-   -   1. LMU detects the frequency control channel (FCCH), which is        used to correct for frequency offset in the BTS.    -   2. LMU detects the synchronization channel (SCH) to derive        accurate timing. While the LMU knows the frame timing, it does        not know the absolute frame number or which cell is transmitting        (CGI).    -   3. LMU demodulates the broadcast control channel (BCCH) and        decodes the frame number and the CGI. Note that demodulation of        the signal relies upon a significantly higher SNR than Steps 1        and 2 above, since detection is easier than demodulation.    -   4. This absolute frame timing reference is available for LMUs to        aid in the collection of signals in the U-TDOA location process.

The Enhanced Beacon Synchronization builds on the basic BeaconSynchronization process as follows:

-   -   1. The AMS provides multiple measurements for each BTS on which        the enhanced process is needed. These measurements include the        CGI.    -   2. Correlation of these messages is performed to derive a        reduced frame number (16 out of the 22 bits needed to represent        the full frame number) and a rough estimate of the timing        synchronization between GSM frames and GPS time.    -   3. The reduced frame number (16 out of 22 bits) information is        sent to the LMU, which then performs the FCCH and SCH detections        (steps 1and 2 above) and returns the accurate time alignment.        Since the LMU no longer has to demodulate the BCCH channel,        detection can be performed at lower SNRs.    -   4. The accurate timing provided by the LMU is combined with the        reduced frame number previously computed. This Frame Number is        then available to aid in the signal collection by the LMUs for        location.

Downlink only LMU Deployments (Step 311 of FIG. 3C)

One technique for increasing the location system performance in DownlinkBeacon Discovery limited areas is the targeted installation ofdownlink-only LMU units. A low-cost, easily-installed receiver unit isdeployed to measure downlink beacon timing in cell sites without an LMUdeployed using the process shown in FIG. 4. By decreasing the number ofLMUs deployed and installing units that provide beacon discovery (butare not capable of cooperating in location processing) to fill in beaconcoverage “holes”, a lower overall system cost is attained. This unit mayuse a wired or wireless backhaul to the SMLC or to another LMU forforwarding to the SMLC. The backhaul is irrelevant to the timingcollection, so variable link latencies will not affect operation.

The downlink LMU can be used to collect raw timing data or be deployedwith a GPS module allowing for an offset from GPS system to be locallycalculated. Deployment with a GPS module simplifies the process ofproviding beacon timing offsets, but the GPS module increases cost andsize of the unit and restricts mounting to areas with GPS coverage. Useof a local clock source or offsets from other beacons allows eliminationof the GPS module and saving the cost of the GPS receiver and GPSantenna; allows more flexible mounting options (should be able to detectcommon beacon to deployed LMU) but relies upon the SMLC to calculate thebeacon timing offset from a common observed beacon.

All the beacons could be timed relative to each other, or to some otherarbitrary time. This timing approach would be fine for the purposes ofbeacon discovery, but does not generate enough timing accuracy forU-TDOA measurements, so accurate GPS-based timing is still required atLMU sites. Beacon discovery can be off by many micro-seconds and stillbe functional, but accurate U-TDOA requires ˜25-50 nanoseconds clockaccuracy for U-TDOA calculations.

The downlink LMU unit may use an internal antenna, but can support anexternal antenna for added gain in situations where an insufficientnumber of beacons are received.

The typical unit's receiver system is single-channel with support formultiple antennas to allow diversity reception. In keeping with themulti-band deployments, the units are capable of tuning across multiplefrequency bands. The receiver unit is designed to support downlinkreception only but can be coupled with a wireless transceiver to allowwireless backhaul. The unit is dedicated to beacon search and reportingof beacon timing or beacon offset timing from a local clock or inrelation to another beacon.

In very limited cases, using knowledge of the wireless communicationsnetwork gained in the initial analysis, it is possible to use theco-synchronous clusters to map beacon timing from a beacon that an LMUcan detect to a beacon an LMU cannot detect but is known to beco-synchronous to a detected beacon. This often applies to sectorswithin a cell site but could also apply to other groups as well if BTSsynchronization via a common clock is used.

-   -   Improving Uplink Demodulation Limited Performance (See FIGS. 3F        and 3G)

If the preliminary network design analysis (Step 303, FIG. 3A) showsthat the sparsed U-TDOA deployment is Uplink Demodulation Limited, fivetechniques can be used to mitigate or correct Uplink DemodulationLimited Performance. The first of these techniques (Step 319, FIG. 3F)requires that the wireless network be configured to forward radiomessages or bit sequence(s) from the reverse channel (mobiledevice-to-Base Station) radio messaging for re-modulation to thewireless location system into a representative signal for signalcorrelation processing.

The second technique (Step 318, FIG. 3F) requires an AMS or LMS andextract bit-sequence(s) from the reverse channel radio messaging forre-modulation into a representative signal for signal correlationprocessing.

The third technique (Step 317, FIG. 3F) avoids the need for signaldemodulation and passive monitoring by using only a known bit-sequencein the radio messaging for signal correlation processing.

The fourth technique (Step 320, FIG. 3G) for combating UplinkDemodulation Limited areas is the addition of dedicated antenna, oraddition of signal processing that combine inputs from all availableantenna, to LMUs within or in proximity to the affected geographic area.LMUs are selected based on the predicted improvement in SNR for pointswithin the affected area and the orthogonality of the resultant TDOAhyperbola(s).

The fifth technique (Step 308, FIG. 3G) is to add LMU(s) within or inproximity to the area where the current TDOA design is UplinkDemodulation Limited. The LMU location is selected based on the datadeveloped from the system design, planning and evaluation tools andmodels previously described.

Forwarded Demodulated Data_(FIG. 3F, Steps 316 and 319)

As introduced above, the wireless communications network can sample andforward bit sequences that occur on the reverse control and/or trafficradio channel.

This bit sequence or sample is then modulated to produce a basebandsignal. This re-modulated baseband signal is then used as the referencesignal. The reference signal can then be correlated against the recordedreceived signal at receiving sites where LMUs are deployed to provideTDOA estimates between the ideal reference and received signal at thosesites. (See U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular TelephoneLocation System.” and U.S. Pat. No. 6,047,192, Apr. 4, 2000, “RobustEfficient Localization System”.)

This technique is especially suited to instances where the LMUfunctionality has been incorporated into a base station transceiver thebit sequence is quickly delivered using the internal BTS communicationspaths. The integration of the LMU with the wireless communicationssystem eliminates the need and cost of the standalone passive monitoringdevice from the wireless location system deployment.

Link Monitoring for Improving Uplink Performance (FIG. 3F, Steps 310 and318)

As discussed above, a Link Monitoring Subsystem (LMS) can be used tosupply a passive means of triggering the location system. The LMS systemalso allows for certain techniques allowing for lower LMU deploymentdensities by improving the uplink demodulation performance. In anon-sparse U-TDOA deployment, the reference signal is normally producedby the LMU resident in the serving cell or an LMU resident in anadjacent cell. In sparsed deployments, no LMU may be able tosuccessfully receive a signal with sufficient quality to be demodulatedwith minimal errors. In this case, the LMS (or AMS) may be used tocapture a bit sequence sample that was included in the signal. Thissample is then re-modulated to produce a baseband signal. Thisre-modulated baseband signal is then used as the reference signal. Thereference signal can then be correlated against the recorded receivedsignal at receiving sites where LMUs are deployed to provide TDOAestimates between the ideal reference and received signal at thosesites. (See U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular TelephoneLocation System.”) Again, as noted above, obtaining the demodulationdata from an AMS or other link monitoring system can reduce thecost/complexity of the LMU, which is advantageous even when sparsing isnot an issue.

Known Sequence Correlation for Improving Uplink Performance (Step 317,FIG. 3F)

The successful measurement of TDOA values requires a “clean” (high SNR,low phase noise, low interference, etc.) reference signal with whichmeasured signals from multiple sites are correlated to provide anestimate of the TDOA between the reference signal and the signalreceived at each site. This reference signal is typically acquired inone of two ways in a non-sparsed U-TDOA network. The first approach isto use the received signal at a site that is close to the mobile (e.g.,the serving cell site) as the reference signal. This approach assumesthat the link budgets are such that the received signal at close-bysites is also fairly clean (uncorrupted by radio interference or noise).The second approach is to reconstruct an ideal reference signal bydemodulating (and if necessary, decoding) the received signal at onesite, then using this data to generate the expected waveform at thereceived site. This approach assumes the signal is received at one ormore sites with sufficient quality to be demodulated with minimalerrors.

In the case of a sparsed LMU deployment, it is possible that neither ofthese approaches will provide an adequate reference signal. Thisscenario can be caused by the mobile being at a location such that noLMUs receive a high quality signal from the mobile station. In thiscase, the first approach results in a low SNR signal that does not serveas a good reference signal. Since the quality of the signal is poor atall sites where LMUs are deployed, the second approach of reconstructinga reference will also fail because the poor quality signal cannot bereliably demodulated (likely to have many bit errors).

Many waveforms, however, have known patterns (e.g., Training SequenceCode in Mid-amble of GSM, Synch and DVCC in IS-136, etc.) that aretransmitted along with the unknown user data to aid in acquisition,synchronization, and/or equalization. With prior knowledge of thesepatterns, an ideal reference can be generated that represents theexpected received waveform associated with these known fields. Thisreference can then be correlated against the received signal atreceiving sites where LMUs are deployed to provide TDOA estimatesbetween the ideal reference and received signal at those sites. (SeeU.S. Pat. No. 6,047,192, Apr. 4, 2000, “Robust, Efficient, LocalizationSystem.”)

D. Conclusion

The true scope the present invention is not limited to the presentlypreferred embodiments disclosed herein. For example, the foregoingdisclosure of a presently preferred embodiment of a Wireless LocationSystem uses explanatory terms, such as Signal Collection System (SCS),TDOA Location Processor (TLP), Applications Processor (AP), LocationMeasuring Unit (LMU), and the like, which should not be construed so asto limit the scope of protection of the following claims, or tootherwise imply that the inventive aspects of the Wireless LocationSystem are limited to the particular methods and apparatus disclosed.Moreover, as will be understood by those skilled in the art, many of theinventive aspects disclosed herein may be applied in location systemsthat are not based on TDOA techniques. For example, the invention is notlimited to systems employing SCS's constructed as described above. TheSCS's, TLP's, etc. are, in essence, programmable data collection andprocessing devices that could take a variety of forms without departingfrom the inventive concepts disclosed herein. Given the rapidlydeclining cost of digital signal processing and other processingfunctions, it is easily possible, for example, to transfer theprocessing for a particular function from one of the functional elements(such as the TLP) described herein to another functional element (suchas the SCS) without changing the inventive operation of the system. Inmany cases, the place of implementation (i.e., the functional element)described herein is merely a designer's preference and not a hardrequirement. Accordingly, except as they may be expressly so limited,the scope of protection of the following claims is not intended to belimited to the specific embodiments described above.

1. A computer-implemented method for designing a sparse wirelesslocation system (WLS) from an initial network design, wherein the sparseWLS is designed to be overlaid on a wireless communications system,comprising: performing an intelligent WLS network design process toproduce an initial network design; storing the initial network design ona non-transitory computer readable medium; determining that the initialnetwork design is affected by an accuracy performance limiting factor;modifying the initial network design by performing at least one of thefollowing actions: increasing an integration time at one or morelocation measuring units (LMUs); deploying a hybrid time difference ofarrival (TDOA)/ enhanced cell identification (ECID) location process inthe WLS; deploying a hybrid time TDOA/ angle of arrival (AoA) locationprocess in the WLS; and adding at least one LMU to the network design;and saving the modified network design on the computer readable medium;wherein said WLS comprises an uplink time difference of arrival (U-TDOA)system including a plurality of geographically dispersed locationmeasuring units (LMUs); and wherein said WLS is overlaid on a wirelesscommunications system comprising a plurality of geographically dispersedbas e transceiver stations (BTSs).
 2. A method as recited in claim 1,further comprising determining that the initial network design or amodified version thereof is affected by a downlink beacon discoveryperformance limiting factor.
 3. A method as recited in claim 2, furthercomprising the following step based on the determination that theperformance limiting factor is downlink beacon discovery: deploying atleast one enhanced downlink antenna.
 4. A method as recited in claim 2,further comprising the following step based on the determination thatthe performance limiting factor is downlink beacon discovery: deployingdownlink interference cancellation.
 5. A method as recited in claim 2,further comprising the following step based on the determination thatthe performance limiting factor is downlink beacon discovery: deployingbase transceiver station (BTS) synchronization.
 6. A method as recitedin claim 2, further comprising the following step based on thedetermination that the performance limiting factor is downlink beacondiscovery: adding at least one location measuring unit (LMU) to thenetwork design.
 7. A method as recited in claim 2, further comprisingthe following step based on the determination that the performancelimiting factor is downlink beacon discovery: determining that an Abismonitoring system (AMS) is not deployed, and then deploying at least onedownlink-only location measuring unit (LMU) at an identified site.
 8. Amethod as recited in claim 2, further comprising the following stepbased on the determination that the performance limiting factor isdownlink beacon discovery: determining that an Abis monitoring system(AMS) is deployed, and then enabling the use of Enhanced BeaconSynchronization (EBS) and AMS-derived beacon timing functions.
 9. Amethod as recited in claim 1, further comprising determining that theinitial network design or a modified version thereof is affected by anuplink demodulation performance limiting factor.
 10. A method as recitedin claim 9, further comprising the following step based on thedetermination that the performance limiting factor affecting the initialnetwork design is uplink demodulation: determining that communicationssystem demodulation data is enabled, and enabling a demodulated datafeature.
 11. A method as recited in claim 9, further comprising thefollowing step based on the determination that the performance limitingfactor affecting the initial network design is uplink demodulation:determining that communications system demodulation data is not enabled,and determining that an Abis monitoring system (AMS) is not deployed andenabling a mid-amble only correction feature.
 12. A method as recited inclaim 9, further comprising the following step based on thedetermination that the performance limiting factor affecting the initialnetwork design is uplink demodulation: determining that communicationssystem demodulation data is not enabled, and determining that an Abismonitoring system (AMS) is deployed and enabling an AMS-deriveddemodulated data feature.
 13. A method as recited in claim 9, furthercomprising the following step based on the determination that theperformance limiting factor affecting the initial network design isuplink demodulation: adding at least one location measuring unit (LMU)to the network design.
 14. A method as recited in claim 9, furthercomprising the following step based on the determination that theperformance limiting factor affecting the initial network design isuplink demodulation: adding at least one dedicated antenna facility tothe network design.
 15. A computer readable medium comprising computerreadable instructions for carrying out a prescribed method for designinga sparse wireless location system (WLS) from an initial network design,wherein the initial network design is produced by an intelligent networkdesign process, wherein said WLS comprises an uplink time difference ofarrival (U-TDOA) system including a plurality of geographicallydispersed location measuring units (LMUs), and wherein said WLS isoverlaid on a wireless communications system comprising a plurality ofgeographically dispersed base transceiver stations (BTSs), said methodcomprising: determining that the initial network design is affected byan accuracy performance limiting factor; and modifying the initialnetwork design by performing at least one of the following actions:increasing an integration time at one or more location measuring units(LMUs); deploying a hybrid time difference of arrival (TDOA)/ enhancedcell identification (ECID) location process in the WLS; deploying ahybrid time TDOA/ angle of arrival (AoA) location process in the WLS;and adding at least one LMU to the network design.
 16. A computerreadable medium as recited in claim 15, wherein said method furthercomprises determining that the initial network design or a modifiedversion thereof is affected by a downlink beacon discovery performancelimiting factor.
 17. A computer readable medium as recited in claim 16,wherein said method further comprises the following step based on thedetermination that the performance limiting factor is downlink beacondiscovery: deploying at least one enhanced downlink antenna.
 18. Acomputer readable medium as recited in claim 16, wherein said methodfurther comprises the following step based on the determination that theperformance limiting factor is downlink beacon discovery: deployingdownlink interference cancellation.
 19. A computer readable medium asrecited in claim 16, wherein said method further comprises the followingstep based on the determination that the performance limiting factor isdownlink beacon discovery: deploying base transceiver station (BTS)synchronization.
 20. A computer readable medium as recited in claim 16,wherein said method further comprises the following step based on thedetermination that the performance limiting factor is downlink beacondiscovery: adding at least one location measuring unit (LMU) to thenetwork design.
 21. A computer readable medium as recited in claim 16,wherein said method further comprises the following step based on thedetermination that the performance limiting factor is downlink beacondiscovery: determining that an Abis monitoring system (AMS) is notdeployed, and then deploying at least one downlink-only locationmeasuring unit (LMU) at an identified site.
 22. A computer readablemedium as recited in claim 16, wherein said method further comprises thefollowing step based on the determination that the performance limitingfactor is downlink beacon discovery: determining that an Abis monitoringsystem (AMS) is deployed, and then enabling the use of Enhanced BeaconSynchronization (EBS) and AMS-derived beacon timing functions.
 23. Acomputer readable medium as recited in claim 15, wherein said methodfurther comprises determining that the initial network design or amodified version thereof is affected by an uplink demodulationperformance limiting factor.
 24. A computer readable medium as recitedin claim 23, wherein said method further comprises the following stepbased on the determination that the performance limiting factoraffecting the initial network design is uplink demodulation: determiningthat communications system demodulation data is enabled, and enabling ademodulated data feature.
 25. A computer readable medium as recited inclaim 23, wherein said method further comprises the following step basedon the determination that the performance limiting factor affecting theinitial network design is uplink demodulation: determining thatcommunications system demodulation data is not enabled, and determiningthat an Abis monitoring system (AMS) is not deployed and enabling amid-amble only correction feature.
 26. A computer readable medium asrecited in claim 23, wherein said method further comprises the followingstep based on the determination that the performance limiting factoraffecting the initial network design is uplink demodulation: determiningthat communications system demodulation data is not enabled, anddetermining that an Abis monitoring system (AMS) is deployed andenabling an AMS-derived demodulated data feature.
 27. A computerreadable medium as recited in claim 23, wherein said method furthercomprises the following step based on the determination that theperformance limiting factor affecting the initial network design isuplink demodulation: adding at least one location measuring unit (LMU)to the network design.
 28. A computer readable medium as recited inclaim 23, wherein said method further comprises the following step basedon the determination that the performance limiting factor affecting theinitial network design is uplink demodulation: adding at least onededicated antenna facility to the network design.